Temperature graphic

Local plant ecologist Richard Alward long has been interested in the phenomenon of nighttime temperatures warming even more than daytime ones due to climate change, working with other researchers back in the 1990s on a study of resulting impacts on vegetation growth.

As a result, he was intrigued by a Sept. 13 New York Times story analyzing the trend as it applied to nearly 250 locations around the country. The story had a twist, focusing on what’s happened when it comes to abnormally warm nighttime low temperatures. It concluded that there’s a clear trend that nighttime temperatures “that would have been unusually hot in the 1960s are increasingly common.”

Alward said he hadn’t previously thought of looking at nighttime temperature trends from the perspective of abnormally warm or cold nights, as opposed to just shifts in average nighttime low temperatures. Since the Times interactive tool containing data for individual cities didn’t include anything for Grand Junction, Alward did the data analysis himself, using the same data source the Times used, the National Oceanic and Atmospheric Administration’s Global Historical Climatology Network.

Alward’s conclusion: When applying the 1960s standard for what constituted an abnormally warm or cold summertime night locally, the decade of the 2010s had more than twice as many abnormally warm nights — 52 — than was the case in the 1960s, which had 25 abnormally warm nights.

Grand Junction had 39 abnormally cold summer nights in the 2010s, compared to 44 during the 1960s, Alward said.

The data is based on readings at the Grand Junction Regional Airport, the location of the official weather data collected by the National Weather Service for the city. The Times, and Alward, defined abnormally cold and warm nights as those respectively below or above the fifth and 95th percentiles of the 1960s distribution for nighttime low temperatures in summer. The percentile thresholds at the time in Grand Junction were 50 and 70 degrees, and Alward used those in the current calculations because what’s abnormal has changed over time and his goal was to compare recent temperatures to prior norms.

Alward said Grand Junction’s average summer nighttime low temperatures increased only 0.7 degrees between the 1960s and 2010s.

“That slight increase in the mean led to a doubling of the number of abnormally warm nights,” he said.

Alward said warmer nights can change the length of a growing season, and more abnormally warm nights can shift things a little more than a change in the average nighttime lows.

He said it’s not so much an issue in Grand Junction, but farther south, where abnormally warm nights might mean 90-degree lows, that can mean people don’t cool off at night and can even die if they’re indoors without air conditioning.

Alward’s data shows Grand Junction dropping, at worst, to a nighttime low of just 78 degrees during one night in the 2010s. That means a night that mostly was in the 80s before dipping lower, which Alward said is likely to be less comfortable but he doesn’t think would be life-threatening.

He said if the average summer nighttime low for Grand Junction moves up another degree and the number of abnormally hot nights doubles again, “it may start getting closer to those thresholds that affect human health but it also means higher electric bills for swamp coolers and air conditioning.”

Researchers long have been monitoring a trend of nighttime temperatures warming more than daytime ones as the climate changes. Cloud cover can contribute to this, trapping heat at night, whereas clouds cool things down during the day. In cities, materials such as asphalt absorb more heat during the day, and as a result cities cool off less at night than other areas.

About five years ago, Joe Ramey, since retired from a career as a forecaster for the National Weather Service, analyzed records all dating back at least a century from 11 western Colorado and eastern Utah sites. He found that while average maximum daily temperatures per decade hadn’t changed much over the last century, average minimum temperatures showed a notable increase. Average minimum temperatures rose from 28.7 degrees during the 1970s to 31.8 degrees for 2011-15, Ramey found.

Alward and other researchers took an interest in nighttime temperature trends in the 1990s. He and other ecologists wondered what effects warmer nights might have on grasslands and shrublands.

“We looked around and nobody was looking at what were the impacts of night warming as opposed to daytime warming or just average warming” on grasslands and shrublands, he said.

He and other researchers looked at climate records and the impacts to a site east of Fort Collins where long-term plant monitoring had been going on. They found that as average nighttime low temperatures rose during the growing season, blue grama, the dominant native grass on the eastern plains, decreased production.

The number of nonnative weeds increased with nighttime warming, and Alward and other researchers theorized that it could be that plants that start growing early in the spring can get started even earlier and use up soil moisture for their growth.

“Then when days are warm enough for grass like grama to grow rapidly, the soil water’s been depleted,” Alward said.